Electric attenuating device



Patented Oct. V10, 1950 ELECTRIC ATTENUATING Ben Seeker, London, England, assigner to Inter national Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application August 2, 1946, Serial No. 687,874 In Great Britain July 10, 1945 secuuu 1, Public Law 69o, August s, 1946 Patent expires July 10, 1965 7 Claims.

The present invention relates to high frequency electric attenuating devices.

At frequencies up to several hundred kilocycles per second at least, there is not much difficulty in constructing fixed or variable electric attenuators from resistance spools, since at these comparatively low frequencies resistances of small or zero phase angle are relatively easily made. At frequenciesabove 1 megacycle per ysecond difl'iculties begin to arise because the reactances associated with the resistance spools and other stray reactance effects associated with conductors and screens become so large that it is practically impossible to produce an attenuator which will give a desired or even predictable performance. Further diiculties are associated with varying contact resistance in the switches used in adjustable attenuators.

There has therefore been the tendency to abandon the attempt to employ resistances in these frequency ranges, but to employ mutual inductance or capacity devices to obtain the desii-ed changes in current 'or voltage.

The present invention makes a contribution to the problem from a different aspect, and the method was suggested by the phenomena assoelated with defective transmission in high frequency co-axialcables intended for use at frequencies of a few megacycles per second. When the outer conductor of such a cable is constructed of aV helical copper tape, and whenthis tape is surrounded by an outer layer of steel, tape, the cable is sometimes found to exhibitexcessive attenuation caused by the partial or complete separation of the turns ofthe copper tape during the laying of the cable. The effect has been shown to bevmainly due to eddy current loss in the steel tape, since the increase of attenuation is muchY less when the steel tape is absent, but is still appreciable.

According to the present invention, there is provided an electric attenuator comprising a coaxial transmission line, the outer conductor of which has a portion of helical form, the turns of the helix being out of electrical contact with one another. Y

The invention also provides an electric attenuator comprising a co-axial transmission line, part of the outer conductor of which is in the form of a helix with the turns out of relectrical contact with one another, and electrically conducting means surrounding at least part of the helical portion of the said outer conductor for dissipating electrical energy.

The invention wllbe'explained with reference to the accompanying drawing, in.which:

Fig. 1 shows a longitudinal sectional view of an adjustable attenuator according to the invention;

Figs. 2, 3, 4 and 5 show transverse sectional views at A-A, B-B, C-C and D-D, respectively of Fig. 1; and

Fig. 6 shows a modification of part of Fig. 1.

The variable attenuator shown in Figs. 1 to 5 consists of a central copper conductor l co-axially surrounded by a copper tube 2 having a helical slot 3 cut through it for part of its length from E to F. The tube 2 is surrounded by a co-axial assembly of tubes mounted between two bushes 4 and 5 by which it may be slid longitudinally along the tube 2. The assembly consists of a cop'- per tube 6 which is complete without any slot, and in line with this tube is another tube l preferably of high resistance magnetic material of high permeability. This tube has a straight longitudinal slot 8 cut in it from one end to the other. Co-axially surrounding the tube 8 are two more tubes 9 and ID of similar material, the rst of which has a straight longitudinal slot l i cut in it from end to end, preferably though not necessarily diametrically opposite to the slot 8 in the tube 1. The tubes 9 and Il) are supportedV in position between a .flange l2 on the bush 4 and a disc I3 mounted on the tube B. As is well known, high frequency currents are substantially confined to a thin skin on the surface ofthe conductors through which they flow,

and the resistance of the conductor is ultimately practically independent of its thickness. When a high frequency current is supplied between the conductors I and 2 at one end of the attenuator, the current tends to flow longitudinally along the outer surface of the inner conductor and along the inner surface of the outer conductor. When this current reaches the gap produced by the helical slot in the conductor 2 it is obliged to flow round the edge of the gap and up on to the outer surface of the tube, and then round a turn of the helix on the outside until it can reach the opposite side of the gap when it returns to the inside surface and proceeds longitudinally to the next gap. There is accordingly effectively a sheet of current flowing round the outside surface of resistance of the outer tube.

Since the tube 'l has a longitudinal slot, the current sheet induced on the inner surface can only cross the gap byV coming up to the outside surface at the gap and flowing round the outside.

to the other'. side of the gap. This induces a further sheet of current in the tube 9 which behaves iu the same way and finally induces a sheet of current on the inside surface of the tube IB which has no gap.

It is evident that any number of co-axial tubes like l' or 9 with longitudinal gaps could be provided one outside the other. An additional power dissipation is produce by each of such tubes, so that very large attenuations can be produced with quite short tubes. preferably of a high permeability magnetic material to increase the skin resistance.

It will be noted that part of the helix is surrounded by the copper tube 6, and the remainder by the system of tubes l, 9, l0 etc. The dissipation per centimetre produced by the copper tube is much less than that produced by this system of tubes, and so by sliding the tube assembly along the tube 2, a very convenient adjustment of the attenuation may be produced.

Qn account of the method by which the power loss is introduced, itis found that the attenuator The tubes 1, 9, l0 etc., are l behaves substantially in the same way as a coaxial cable; the attenuation varies at the squareroot of the frequency, and the characteristic impedance is substantially the same as that of the co-axial line formed by the conductors i and 2 without the helical slot. The attenuator is thus substantially equivalent to a long length of coaxial cable.

rI he attenuation per unit length may be calculated from the following formulae, assuming that the widths of the slots are small enough to be negligible. The symbols used are as follows:

r1=outer radius of conductor 1 r2=inner radius of conductor 2 r3=outer radius of conductor 2 r4=sum of all the inner and outer radii of the surfaces of the tubes 7, 9, 19 etc., over which any current sheets iiow.

H1, p1=permeability and resistivity of copper tubes.

n2, p2=permeability and resistivity of the tubes 7, 9,

10 etc.

p=\/7g=ratio of the surface resistivities of the lLupi copper and magnetic material. a=angle of the helix with respect to the axis. A=attenuation per unit length of the tube system. A0=attenuation per unit length of the co-axial cable formed by conductors l and 2 without any slot.

Then For the portion of the helix surrounded by the copper tube E, 10:1 and r4=5, the inner radius of the tube 6. The attenuation per unit length of this portion of the attenuator being a, the increase produced by the tubes of magnetic material will be given by formulae, and accurate formulae for calculating it will be less simple.

In many cases, of course, fixed attenuators will be required, in which case it is not necessary to provide a sliding assembly, and the tube B is not wanted. The helical slot will then be arranged wholly inside the assembly of the tubes 1, 9, Ill, etc. In the case of a fixed attenuator, it may however be desirable to provide for a small adjustment, and Fig. 6 shows a, modification of the left hand end of Fig. 1 to illustrate this.

A. leWed copper sleeve I4 having a knurled head slips over the copper tube 2 and is screwed into the bush 4. The sleeve carries a short tubular extension i5 which surrounds part of the slotted portion of the tube 2. It will be evident from the explanation given above that the attenuation per centimetre corresponding to the portion of the helix screened by the tube I5 will be less than that corresponding to the rest of the helix, so that by screwing the tube I5 in or out a very fine vadjustment of the attenuation will be obtained.

It will be noted that since the attenuator according to the invention is closely equivalent to a length of co-axial cable, it may be directly employed as an attenuation equaliser when connected in the negative feedback path of an amplifier used on the cable.

It will be evident that adjustment of the attenuation could alternatively be effected by stretching or compressing the tube 2 longitudinally, which would change the angle a of the helix and also the effective length of the attenuating portion. While the helix may readily be constructed by cutting a helical slot in a solid tube, it might also be formed by winding copper tape or strip into a helix.

What is claimed is:

1. An electric attenuator comprising a coaxial transmission line, the outer conductor of which has a helical slot therein, and two or more metallic tubes of different diameter coaxially surrounding at least part of the slotted portions of the outer conductor, said metallic tubes being arranged one inside the other and each of them except the outermost having a longitudinal slot therein.

l 2. An electric attenuator according to claim 1 comprising a metallic screening tube arranged co-axially inside the innermost metallic tube and adapted to screen a portion of the helical slot from the innermost metallic tube.

3. An electric attenuator according to claim 2 in which means arranged between said screening tube and said other tubes is provided for longitudinally adjusting the screening tube with respect to the other metallic tubes.

4. An electric attenuator according to claim 1 in which the said metallic tubes of different diameters are constructed of magnetic material.

5. An electric attenuator comprising a coaxial transmission line, the outer conductor of which has a helical slot therein, two or more metallic tubes of dilferent diameter coaxially surrounding at least part of the slotted portion of the outer conductor, said metallic tubes being arranged one inside the other and each of them except the outermost having a longitudinal slot therein, and an additional metallic tube ccaxially sur rounding a different part of the slotted portion of the outer conductor.

6. An electric attenuator according to claim 5 comprising means for adjusting the length of the 5 Slotted portion surrounded by the said tubes of dierent diameters.

7. An electric attenuator according to claim 6 in which the additional tube is fixed axially in line with the said metallic tubes of different diameters to form an assembly adapted to be slid longitudinally along the said outer conductor.

BEN SECKER.

REFERENCES CITED The following references are of record in the le of this patent:

v Number Y 6 UNITED STATES PATENTS Name Date Herbst Mar. 16, 1937 King et a1 1 Mar. 21, 1939 Roosenstein Apr. 8, 1941 Goldmann Aug. 24, 1943 

